I bought Serious Sam 3 BFE and the DLC "Jewel of Nile", only. There are many free modifications, free extra custom maps and free extra custom campaigns... the other SS games are pretty old and probably not worth your time unless you like single-player mode.

The First Encounter and The Second Encounter are, by far, my favorite Serious Sam games of the moment. Serious Sam 3 is good, though. If you don't mind lesser graphics, you can snag the originals for only $9.99 by upgrading Serious Sam 3 to the Digital Deluxe edition. Or you can buy the HD remakes separately.

There are some small gravity-related issues in which some of the "special rooms" from the originals do not work as they originally did in the HD remakes, but it's no big deal, I suppose.

It's tough to give an estimate on the amount of players you're going to find in any of the games at any given time. Doing a random check, my cooperative server list in SS3 returns 8 people playing. Versus shows 2. The First Encounter (HD) returns no players. The Second Encounter (HD) returns no players. The First Encounter (Original) returns 3 players. The Second Encounter (Original) returns 7 players.

Of course, it all depends. All of the games are a blast if you play single player, or with friends. I was never much of one to enjoy playing cooperative with randoms, anyway.

either Episode 1 or Serious Sam 3. Those two are the best, and I'm not sure which one wins out for me. The ancient Egypt vibe is lovely, though.

SSe1 is definitely worth your time. SS is primarily a sp franchise, is it not? I never even play online mp in FPSs, since it's basically been the same experience since DOOM, and I got that experience out of my system almost a decade ago.

Coop is always preferable to me, and SSe1 has coop, and it's very fun to play.

You'll be missing out :PIf you wanna get into What Happen after Sam enters the Time lock, then Serious Sam The First Encounter & Serious Sam The Second Encounter for you. Those first two are Epic with much Much MUCH Challenges of what FPS should be. They more glorified into Battle more than SS3. I'm talking about serious Bossfights, Waves and Waves of Enemies more than what SS3 can bring. SS3 more into Mods & Campaigns now, now that it got it's own Workshop :D which that can satisfy your interest more into the game originally. For example Alien Replacements with stronger and more Serious Ones from Workshop , Weapons Modifications, Add-On Maps......... Just imagine the different Experience and the Time extension into the game with those Mods / Maps

The SSHD Second Encounter got lots of mods and campaign too that can be downloaded separately that can blow your mind example the Forgotten City still Stain unto the Minds of players. That Map was created by Solais with lots of EPic scenry and surprises, actions and lots more. SSHD The First Encounter carries on into The Second Encounter as a Fusion DLC where you can enjoy Mods on the Egypt Episode as well.

Serious sam 3 and Serious Sam TSE are active online but i cannot give an accurate figure on players that are online. Keep in mind that also there are talented modders got lots of Great and Awesome Maps available for Online Coop / Survival and Deathmatch too.

Serious Sam 2 is Awesome. No Egypt but more into Sci Fi Mumbo Jumbo :) And it is Funny

I will recommend ALL on my behalf but for your question, I will say Serious Sam HD The First and HD The Second Encounter and Serious Sam 3

Personaly, I recommend Serious Sam Second Encounter (Classic). I played through all of the Serious Sam games, and the HD versions don't feel as fun to me as the classical ones, although that may just be my nostalga and love for the gravity efffects in them. Anyways, hope you enjoy any of the SS games as much as I have!